1. Field of the Invention
This invention relates to a method for the surface treatment which forms a layer of the carbonitride of vanadium (V) or chromium (Cr) on the surface of any of such articles made of iron or an iron alloy as dies, jigs, tools and machine parts.
2. Description of the Prior Art
It is well known that if a layer of the carbide, nitride or carbonitride of vanadium or chromium is formed on the surface of an article made of iron or an iron alloy, it improves various properties of the article, including its resistance to wear, seizure, oxidation and corrosion. Therefore, there have so far been proposed a variety of methods which are intended to form such a surface layer.
For example, Japanese Laid-Open Patent Specifications Nos. 200555/1982 and 197264/1983 propose immersing an article of an iron alloy in a bath of molten chloride to form a layer of chromium carbide on its surface. Japanese Patent Publication No. 24697/1967 and United States Patent No. 4,242,151 propose nitriding an object of an iron alloy and chromizing it to form a layer of chromium carbonitride on its surface.
According to any of these methods, however, the article is heated at a temperature which is higher than the A.sub.cl transformation point of iron which is about 700.degree. C. The heat is likely to develop in the article the stress which causes it to crack if it has a complicated shape. Moreover, it worsens the working environment, because treatment is done at high temperatures.
There have also been proposed methods which employ a temperature which is lower than about 700.degree. C. They include CVD (chemical vapor deposition), PACVD (plasma assisted chemical vapor deposition) and PVD (physical vapor deposition employing halides of vanadium and chromium, as disclosed in, for example, Japanese Laid-Open Patent Specifications Nos. 653457/1980 and 154563/1980. These methods can form a surface layer without developing any thermal distortion in the article, as they employ a heating temperature which is lower than the A.sub.cl transformation point of iron. It is, however, difficult to form by any of those methods a layer having a uniform thickness and adhering closely to the surface of the article. They involve a complicated process which requires expensive facilities. Moreover, they require the presence of hydrogen or a reduced pressure which lowers the efficiency of the operation.